1. Field of the Invention
This invention pertains generally to bridged networks. More particularly, the invention is a method and apparatus for reducing flooding in a bridged network.
2. The Prior Art
Bridges are generally used for connecting multiple LANs (local access networks) as is known in the art. Bridges operate in the data link layer of the OSI (Open System Interconnection) reference model. As such, bridges do not examine the network layer header and are thus capable of copying IP, IPX and OSI packets equally well.
Bridges are useful in a plurality of situations, including connecting LANs which are separated by considerable distances and splitting a single LAN into separate LANs to accommodate large bandwidth load, for example. Bridges are also useful for connecting LANs of different types, such as connecting an 802.3 LAN to an 802.4 LAN.
Bridges use source and destination MAC address information to determine how and where to forward packets. For example, in a three-port bridge, the bridge inspects MAC information in a packet to determine which of the three ports to forward the packet to.
In operation, a bridge maintains a “bridge table” containing a mapping from MAC address to port. Initially the bridge table is empty; the bridge adds entries during operation. Typically, when the bridge receives a packet, the bridge inspects the destination MAC address and determines from the bridge table which port to forward the packet to. If the destination MAC address is not associated with an entry/port in the bridge table, the bridge broadcasts (or “floods”) each port, thereby forwarding the packet to each port. When a reply is received from the destination node, the bridge determines from which port the reply originated, and establishes the map between the destination MAC address and the port. After this mapping is established, future packets directed at the same destination MAC address are not broadcast to every port, but only to the port which is mapped to the address.
Traditional bridging performance is acceptable in most cases. However, in cases where there is a large amount of traffic on the LANs connected by the bridge, several drawbacks result from excessive flooding of the ports (and therefore the LANs attached to the ports) of the bridge. The primary drawback with excessive flooding is the reduced bandwidth available to other devices on the LANs to the point where, for example, critical applications may not be able to access necessary resources. Other drawbacks from excessive flooding are also known in the art, including, for example, dropped packets.
In addition, where a bridge connects a network over a WAN (wide area network) link, the costs for communicating over the WAN link increase as the number of broadcast flooding communications increase, thereby increasing the cost associated with running the bridged network.
Accordingly, there is a need for a method and apparatus which reduces or controls flooding in bridged networks. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the background art.